this.
On this very forum, we make extensive use of categories – we do use groups too, but only for very specific purposes. Most of the people here are not part of any custom groups.
Very large forums can run effectively with limited or no use of groups.
That said, they are valuable – especially for organizations or communities with greater complexity or who limit access to certain areas for different reasons – but that’s stuff we believe is worth investing more in if you need it (whether your time or your money).